The Correct Way to Partition Raid 0 Hard Drives
By Ellis Davidson
RAID and partitioning are both methods of creating logical hard drives which are different sizes than the physical drives on which they are stored. There is only one correct way to partition drives used for RAID storage.
Hard Drive Partitions and RAID
Partitioning is a kind of hard drive reformatting, which sets up the drive to be used as one or more "logical drives." Logical drives are what the operating system sees, and what you actually use; the physical drive they are stored upon contains them.
As an analogy, consider a large, empty tract of land which is planned for use as residential housing and farming. The housing and farms can be considered to be the "operating system" and "applications" stored on that land, but before they can be built, a surveyor must first divide the land into partitions which determine where the houses and farms will go. If you want to move the farms later, you'll need a new surveyor to repartition the land, which can be extremely inconvenient if you need to tear down houses to build new farms.
Most new hard drives are sold with a pre-existing partition: a single logical drive which takes up the entire space of the physical hard drive capacity. Many operating systems also require additional hidden partitions, which are small amounts of storage set aside for internal use. These can be reviewed with low-level disk utilities, but as the space is always used from the point of the view of the user, it's easier to think of them as drive overhead, rather than separate usable partitions.
A physical drive can also be made into two or more logical partitions: for example, a 500 gigabyte physical drive can be partitioned into two logical disks of 300 gigabytes, and 200 gigabytes. Common reasons for doing this are to separate the boot operating system from a logical data drive; both are stored on a single disk, but they appear to the operating system as separate drives.
Meanwhile, RAID reverses this formula: a RAID format creates a "superpartition" which spans multiple drives. RAID 0, for example, can take four 500 gigabyte drives, and make them appear to be a single logical two terabyte drive (4 * 500 = 2,000 gigabytes, or 2 terabytes).
Partitioning a RAID Drive
RAID works by combining several drives, and there is only one way to partition these drives: as a single partition, one logical drive per physical drive. Some RAID systems might allow you to, say, create a RAID across multiple partitions on the same physical drive, but to do so would be to defeat the purpose of a RAID, which is meant to span multiple disks.
However, once you have applied a RAID system to multiple disks, they in turn present themselves as a single logical drive to your operating system, and these can in turn be partitioned. So starting with four 500 gigabyte drives, you would partition each one as a single drive. The RAID 0 system would then create a logical two terabyte drive out of them, at which point, if you wish, you can partition the RAID itself into a logical terabyte drive and two 500 gigabyte drives, or any combination you prefer.
Ellis Davidson has been a self-employed Internet and technology consultant, entrepreneur and author since 1993. He has written a book about self-employment for recent college graduates and is a regular contributor to "Macworld" and the TidBITS technology newsletter. He is completing a book on self-employment options during a recession. Davidson holds a Bachelor of Arts in American civilization from the University of Pennsylvania.